People who do not make any insulin (or very little) have Type 1 diabetes. Because the immune system destroys the pancreas, they have stopped making insulin, and their body is unable to use glucose for energy. They tend to lose weight very quickly because their body is actually being starved. Their health rapidly deteriorates and they would die if insulin were not given.
They therefore require insulin by injection several times each day. Along with some dietary changes, this will allow the person to maintain good health.